Since high-strength and high-modulus polyethylene fiber was produced in 1 980's, intensive study of gel spinning has been made, and it has been found that there are three key factors for producing high-strength and high-modulus polyethylene fiber by gel spinning, i.e., (1) the disentanglement of ultra-high molecular weight polyethylene (UHMW-PE) in solution; (2) the formation of a gel filament and the maintenance of the disentangled state of UHMW-PE; and (3) ultrahigh-thermal stretch, resulting in high crystallinity and orientation of the macromolecular chain of PE, accompanied by the transformation of the PE crystal form. Among others, more attention has been paid to the formation of a gel filament, and actual effects have been achieved.
WO 01/73173A disclosed a process for producing high-strength and high-modulus polyethylene fiber by shear extruding a solution through a thick spinneret with an ultra length/diameter ratio (φ=1 mm, L/D≧40), and performing a jet stretch at an extension rate of more than 500 min−1 and a stretch ratio of at least 5 within a narrow spin gap (where transverse air is applied). However, this process has three disadvantages, i.e., (1) the ultra length/diameter ratio of the spinneret evidently results in the increase of the flowing resistance of the fluid, so that the extruding velocity through the spinneret orifice decreases (to only 1 ml/min), and therefore the spinning efficiency is low; (2) the controllable range of the jet stretch is very narrow (≦6.4 mm), so that in the actual operation, fluctuation of the quench bath surface due to the cycling of the quench bath liquid not only influences the extension rate during the spinning, but also even cause the quench bath to reach the spinneret and thereby cause the spinning to be interrupted; (3) it is difficult to apply the transverse air at a flow rate of 0.76 m/min within the spin gap, and specifically, the transverse air at this flow rate will become insignificant when the number of the spinneret orifice increases.
WO 2005/066401A disclosed another process for producing high-strength and high-modulus polyethylene fiber, the essentials of which is the improvement of the shape of a spinneret orifice. In this process, the spinneret orifice is composed of two portions, i.e., a leading hole and a spinning hole. The leading hole has large diameter and length/diameter ratio (φ=3 mm, L/D=18), while the spinning hole has small diameter and length/diameter ratio (φ=1 mm, L/D=10), and the cone angle from the leading hole to the spinning hole is in the range of 50°˜60°. The long spinneret orifice cause an increased shear stress of the solution, so that the extruded fluid can be stretched easily so as to greatly increase the extension rate of the jet stretch and the thermal stretch ratio of the gel filament, thereby obtaining high-strength and high-modulus polyethylene fiber. However, this process also has three disadvantages, which are (1) the thickness of the spinneret greatly increases due to the incorporation of the long leading hole, so the flowing resistance of the solution increases, and specifically, the maximum volume flow rate for a single orifice is only 2.2 ml/min, which is obviously disadvantageous for an effective spinning; (2) a jet stretch produces effect at a high stretch ratio (the stretch ratio of 40 in the Example 1.2), but such high stretch ratio would endanger the stretch stability; (3) if the jet stretch ratio decreases, the thermal stretch of the gel filament will become difficult in terms of both process and facility.